By \(D(\mathbb {R}^{n})\)  we shall denote the linear space of compactly supported infinitely differentiable functions;
the topology on this space is standard (see, for example, Ch. 6 of [1]}). Next, \(D^{\prime }(\mathbb {R}^{n})\)  will denote the linear space of continuous linear functionals on
\(D(\mathbb {R}^{n})\) . Following [2]} we shall be concerned only with functionals from the space
\(S_{e}(\mathbb {R}^{n}) \subset D^{\prime }(\mathbb {R}^{n})\) . Note that \(f \in S_{e}(\mathbb {R}^{n})\)  if and only if
\(|\langle f,\varphi \rangle | \le C_{f}\sup \lbrace |D^{\alpha }\varphi (x)|\operatorname{exp}(N_{f}|x|),x \in \mathbb {R}^{n}, |\alpha | \le N_{f}\rbrace , \quad \varphi \in D(\mathbb {R}^{n}).\) 
